Low voltage protective device



Sept. 8, 1942. E. F. w. BECK EIAL 2,295,379

LQW VOLTAGE PROTECTIVE DEVICE Filed Feb. 29, 1940 INVENTORS Edward I? W Beck 8 Charles M Zeal:

WITNESSES:

ATTORNEY Patented Sept. 8, 1942 FFlCE LOW VOLTAGE PROTECTIVE DEVICE 1 Edward F. W. Beck, Pittsburgh, and Charles M. Lear, Irwin, Pa., assignors to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application February 29, 1940, Serial No. 321,468

4 Claims. (Cl. 201-76) The present invention relates to protective electric discharge devices and, more particularly, to

an electric discharge device for the protection of low voltage apparatus against voltage surges.

The protective device of the present invention is especially adapted for the protection of low voltage electrical apparatus, such as copper oxide rectifier units and similar devices, against lightning or other voltage surges. The protection of such devices has presented a very difllcult problem since they have a low impulse strength of the order of a few hundred volts, which makes it necessary that the protective device have a low breakdown voltage and a low discharge voltage and, in addition, the protective device must have high resistance at normal operating voltages in order to keep the leakage current resulting from the normal operating voltage to a very low value so as to prevent interference with the normal operation of the protected apparatus. The exist ing types of low voltage protective devices, such as are used for the protection of signal and communication circuits, have been found to be inadequate for the protection of rectifier units and similar apparatus, since they are not capable of holding the discharge voltage down to a low enough value, and in some cases, they do not have high enough resistance to leakage currents at normal operating voltages. V,

The object of the present invention is to provide an electric discharge device for the protection of low voltage apparatus against voltage surges which has a relatively low breakdown voltage and "alow surge discharge voltage, and which has high resistance to leakage currents at normal operating voltages.

A further object of the invention is to provide a low voltage protective device whmllis capable of discharging surges of high voltage and of holding the discharge voltage across the protected device down to a relatively low value.

Another object the invention is to provide a low voltage protective device which is capable of discharging very high surge currents without damage to the device itself or to the protected apparatus.

The protective discharge device of the present inventioninitssimplestformconsistsofapalr,

terial. The dielectric layer is made very thin and should be more or less porous so that a large number of restricted discharge paths is provided between the electrodes and the dielectric layer will not be damaged by the discharge. In the preferred embodiment of the invention, the dielectric layer is provided by placing a thin coating of baked sodium silicate on one of the carbon disc electrodes since such a coating can be made very thin so as to give a low breakdown voltage and is more or less porous. when a plain carbon disc is placed in contact with the coated disc, a discharge devlce is formed which is capable of holdlng voltage surges down to a'low ,value and of discharging very high surge currents. This device also has very high resistance at normal voltages so that the leakage current is held to very low values and it is, therefore, very well adapted for the protection of low voltage apparatus, such as rectifier units, against lightning and other voltage surges.

The invention will be more fully understood from the following detailed description, taken in connection with the accompanying drawing, in which:

Figure l is a transverse sectional view of a protective discharge device with the associated apparatus shown diagrammatically.

Fig. 2 is an exploded perspective view of the discharge device, and

Fig. 3 is a view similar to Fig. 1, showing a further embodiment of'the invention.

Figs. 1 and 2 show a preferred embodiment of the protective discharge device. The discharge device I consists essentially of two electrodes 2 and I which are separated by a thin dielectric layer 4, the thickness of which is greatly exaggerated in the drawing for the sake of clearness. As shown in Fig. l,.the upper electrode 2 is connected to a'line I in which the protected apparatus t is connected, while the lower electrode 3 is connected to ground as indicated at I. The

protected apparatus 0 may be a copper oxide in Fig. 2 and, in its preferred embodiment, it

consists of two thin disc-shaped electrodes 2 and I of low resistance material, which is preferably carbon, and of relatively large area. The upper electrode 1 is a plain carbon disc while the lower electrode 8 has an insulating coating 4 applied to its surface to constitute the dielectric layer which separates the two electrodes. This dielectric layer is made very thin in order to provide a low breakdown voltage, and its thickness is preferably in the range from one-half to mils, the thickness depending upon the desired breakdown voltage. It is undesirable for the dielectric layer to be punctured by the discharge and in order to prevent this, and also in order to provide high surge current capacity, the dielectric layer 4 is preferably made more or less porous, so that a large number of discharge paths are provided.

It has been found that the most suitable material for the dielectric layer 4 is sodium silicate or waterglass, which may be applied to the electrode 3 as a liquid and baked on it to form a thin coating. After baking, the coating can be rubbed down to reduce its thickness to as great an extent as is desired. Such a coating can be made extremely thin and has many fine pores in it. so that the breakdown voltage is low and the pores provide a large number of discharge paths between the electrodes. It is probable that when the protective discharge device operates, a large number of discharges take place simultaneously through the pores of the coating 4, so that a high surge current capacity is obtained, while the existence of the pores in the coating prevents damage to it by the discharge.

Other suitable materials may, of course, be used to form a thin insulating film or coating on one of the electrodes, but inorganic materials are preferable in order to prevent the possibility of charring of the dielectric layer by the discharge with consequent bridging of the electrodes. It is not necessarily essential that the dielectric layer 4 be porous initially, since if a suitable material is used and the layer is made thin enough, the discharge itself will form a plurality of fine pores in the coating rather than puncturing it in only one or a few places of relatively larger area, and thus the operation will be the same as if a porous coating had initially been provided. It is usually preferable to place the insulating coating on one of the electrodes only, since the dielectric layer must be very thin to obtain a low breakdown voltage, but where higher breakdown voltages are desired, a thicker layer may be used and a thin coating may be provided on both of the electrodes in order to obtain the increased thickness.

For higher voltage applications, an increased breakdown voltage may be obtained either by using a thicker dielectric layer between the electrodes, or by assembling a plurality of coated electrodes similar to the electrode 3 in a stack.

Such an embodiment of the invention is shown in Fig. 3, which shows a discharge device 8 consisting of a plurality of carbon disc electrodes 9 placed in a stack and separated by thin dielectric layers It]. Each of the electrodes 9 is preferably similar to the electrode 3 described above so that a discharge device is formed consisting of a plurality of low resistance electrodes of relatively large area separated by thin, porous dielectric layers. The top electrode ll may be a plain carbon disc similar to the electrode 2 previously described.- The assembly thus formed may be connected between a line 12 to be protected and ground I3 or in any other circuit arrangement as required by a particular application. The discharge device 8 has a higher breakdown voltage than the discharge device I shown in Figs. 1 and 2 and is suitable for the protection of higher voltage apparatus or circuits.

It should now be apparent that a low voltage discharge device has been provided which is of simple and inexpensive construction. The new discharge device is very well suited for the protection of low voltage electrical apparatus, such as copper oxide rectifier units and similar devices, against voltage surges and is also well adapted for other low voltage applications, such as the protection of signal and control circuits against lightning. The new discharge device has many desirable characteristics which are not obtainable with the previously used types of low voltage protective devices. Thus, protective devices consisting of electrodes separated by short air gaps cannot be used satisfactorily for the protection of rectifier units, since it is not practical to adjust such devices to have a low enough breakdown voltage to give adequate protection, and discharge devices of the type in which the electrodes are separated by solid dielectric material, such as paper or mica, are not suitable for such applications because the dielectric is punctured by the discharge, thus forming an air gap which has high breakdown voltage. Devices of these types are not capable of discharging high surge currents and usually have relatively high surge discharge voltages. Various other types of low voltage protective devices have also been tried for the protection of rectifier units and have been found unsatisfactory for this purpose.

The new protective device has characteristics which were not obtainable with any of the low voltage discharge devices of the prior' art and which make it very desirable for low voltage protective applications. Thus, it has been found by tests that the new discharge device will keep a voltage surge which would normally rise to 60 kilovolts down to less than 1,000 volts while discharging 10,000 amperes. The leakage current at a normal operating voltage of 20 volts D. C. was only a few rnilliamperes even after the device had been surged while connected to a source of rectified voltage. These characteristics make the new discharge device very well adapted for the protection of very low voltage devices, such as copper oxide rectifier units and similar apparatus, and it is capable of adequately protecting such devices even on repeated surges of kilovolts and 10,000 amperes. These characteristics are not obtainable with any of the low voltage protective devices which have previously been used, none of which are capable of keeping the discharge voltage down to such a low value or of discharging such high surge currents. The relatively large area of the electrodes of the new device, together with the porous dielectric layer which provides a large number of discharge Paths of small cross section insures a very high surge current capacity, and provides adequate protection against bridging the gap because of the small area of each discharge path.

A specific embodiment of the invention has been shown and described for the purpose of illustration but it should be understood that the invention is not limited to the exact construction shown since it is capable of various modifications and embodiments. Thus, any suitable material may be used for the dielectric layer and any necessary thickness of layer may be used to obtain the desired characteristics. The new discharge device may be connected in any suitable circuit arrangement for protection against voltage surges and its usefulness is not limited to the particular types of apparatus mentioned above, but it may be used in general for the protection of any low voltage device or circuit. The invention is not limited therefore to the exact arrangement described above, but in its broadest aspects it includes all equivalent embodiments and modiflca tions which come within the scope of the appended claims.

We claim as our invention:

1. An electric discharge device comprising. a pair oi electrodes of low resistance material separated by a thin porous layer of sodium silicate.

2. An electric discharge device comprising two thin carbon plates placed in contact, the contact surface of at least one oi the plates having a thin porous coating of sodium silicate on it.

3. An electric discharge device comprising two thin carbon plates placed in contact, the contact surface 0! at least one of the plates having a thin coating of baked sodium silicate on it.

stack, each of said plates having a thin coating of 10 sodium silicate on at least one surface thereof.

EDWARD F. W. BECK. CHARLES M. LEAR. 

